It is easy to think for the people that the water on our planet is more than enough. Water covers 70% of our planet. But, despite that, fresh water is an important resource, scarce on Earth. It is only 2.5% of the total. But this tiny amount of fresh water is not all available, 69% is in the glacier and 30% is underground water. Just 1% of this quantity.
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What is desalination?
Seawater has mineral salts that make it brackish and not drinkable for humans, so much so, that if ingested in large quantities can lead to death. Desalination or desalting is the process of removing salt from seawater or brackish water, in order to convert it into drinking water; achieving this is one of the possible Solutions to the shortage of drinking water, since desalination of sea water could provide fresh water for supply and irrigation. These processes may be used for municipal, industrial, or commercial uses.
Desalination plants are industrial facilities for desalination, usually from the sea or salt lakes, to obtain drinking water. Currently seawater desalination plants produce drinking water for many comunities, reaching the amount of over 99 million cubic metres of desalinated water per day worldwide, which provide more than 100 million people with drinking water; but until recently this process was very expensive and therefor reserved for extreme conditions only.
According to a study from the Institute for Water, Environment and Health at the United Nations University (UNU-INWE) in 2019, approximately in 177 countries, there are 16,000 desalination plants in operation. And all these plants generate around 95 million m3/day of fresh water.
The first country to take on this activity was Australia. Many parts of its territory are arid. It has plants in the main cities that operate through reverse osmosis.
The leader of desalination plants by volume is Saudi Arabia, followed by the United Arab Emirates, both of which are desert countries and highly dependent on this process. Other countries, such as Kuwait and Qatar, in the Middle East, have also declined this process.
Water Desalination process
Water desalination processes consist of separating dissolved salts and other minerals from water. Feedwater includes brackish, seawater, wells, surface (rivers and streams), wastewater, and industrial feed and process waters. Membrane separation requires driving forces to overcome natural osmotic pressures and effectively move water through the membrane processes.
The seawater desalination process has the potential to produce large amounts of potable water to support populations located near the coast.
Reverse osmosis (RO) and nanofiltration (NF) are the main pressure-driven membrane processes. The most widely used forms of the membrane are spiral wound, hollow fiber, and spiral sheet. Operating pressures for RO and NF are in the range of 50 to 1000 psig (3.4 to 68 bar, 345 to 6896 kPa).
Electrodialysis (ED) and reverse electrodialysis (EDR) processes are powered through direct current (DC) in which ions flow through selective membranes with oppositely charged electrodes by pressure.
In EDR systems, the polarity of the electrodes is reversed at alternate periods. The anions and cations separate the ions in the feed water.
These processes are carried out in waters with a low level of dissolved solids (TDS).
Forward osmosis (FO) is a fairly new process. Wherein salt concentration (osmotic pressure) is the driving force across a synthetic membrane. Without adding any external force, the water from the feed solution will naturally pass into the extraction solution. The diluted solution is then processed to separate the product from the reusable extraction solution.
Technologies used for desalination
The objectives of desalination are to remove undesirable elements from water, making it accessible for human consumption or for industrial applications. There are different techniques to remove impurities such as In general, different techniques are presented to remove impurities, including microorganisms, suspended solids, organic materials, and other contaminants.
The factors that determine which technique to use are the properties of the water, the cost of treatment, and the anticipated requirements of the treated water.
Using pumps, the water to be treated is extracted from its source or directed to the piping system or storage tanks. To avoid contaminants adjacent to the water, this body structure must be made of excellent materials and assembled in a way that no unplanned contamination occurs.
The first stage of water treatment is to remove large solids, such as sticks, debris, leaves, and other items. Most deep groundwater does not require screening before further purification steps.
The pH values of the sea vary between 7 and 8.4. On the other hand, natural water is around 7. The pH values of the water can change depending on the geology of the aquifer or water basin and have an impact on the participation of contaminants. If the water is acidic (pH value less than 7), sodium hydroxide, lime, or soda ash can be used to raise the pH at some point during the water treatment process. For acidic waters, «forced draft degassers» may be an advantageous method.
Increasing the pH value of the water (alkaline water) helps the «coagulation and flocculation» processes to work properly. Good alkalinity reduces the corrosion of iron pipes in water. Alkaline water (pH greater than 7.0) already ensures that metals, such as copper or lead, will not mix with the water. Calcium carbonate precipitate is a function of different parameters including pH, temperature, mineral content, calcium concentration, and alkalinity.
Coagulation and flocculation
The first step in the vast majority of water treatment processes is the addition of special chemical products to facilitate the removal of components found in the water.
The addition of coagulants such as «aluminum sulfate» or «iron salts» can give incalculable chemical and physical reactions. Through inorganic coagulants, the particles are neutralized in seconds at a very low cost. On the other hand, aluminum and iron ions begin to form precipitates containing metal hydroxide. The metal hydroxide goes to the conglomerate in larger particles. The large molecules of amorphous aluminum hydroxide and iron(III) remove or adhere to suspended particles, facilitating the removal of particles by consistent methods such as filtration and sedimentation.
Several factors vary the quality of a sedimentation system. These factors are the settling rate of the suspended particles, the volume/area of the tank, and the flow rate through the tank. Design settling tanks are calculated within an overflow rate range of 0.5 to 1.0 gallons per minute per square foot (1.25 to 2.5 m per hour).
The efficiency of the tank does not determine the retention time or the depth of the tank. However, the tank must be large enough not to disturb the sludge formed by running water. The typical retention time for the sedimentation process ranges from 1.5 to 4 hours and tank depth ranges from 10 to 15 ft (3 to 4.5 m)
Dissolved air flotation (DAF)
It is used when the suspended particles do not settle easily in the water through the sedimentation process. After the coagulation and flocculation processes, the treated water is directed to the DAF system tanks. Air diffusers at the bottom of the tank are necessary to create air bubbles attached to the floc, creating a suspended buoyant mass. The floating floc blanket is removed from the soil and clean water is drawn from the base of the DAF tank. The DAF system is critical for water-containing algal blooms with low turbidity and high coloration.
Rapid sand filters are the most used type of filters. The water circulates vertically through the layer of sand with a layer of «activated carbon» or «anthracite coal» on top. The first layer removes the natural components, affecting the taste and smell. The space between the sand particles is larger than the smallest particle suspended in the water, so only a small part of the particles are trapped in the separation areas or adhere to the sand particles. The filtration efficiency depends on the amount of sand filter. The filtration rate of the filter is the key to its correct operation.
Another method that can help is to use compressed air to inflate the compacted filter media and drive the air-flushing process.
The mechanism of operation of the sand filter occurs when the water is directed upwards quickly, to remove embedded or unwanted particles.
Pressure filters work on the same concept as fast gravity filters. The main difference is that the filter is inside a container where water is forced under pressure through the filter media. The benefits of gravity filters are:
- Screens have much smaller particles than other sand filters.
- Effectively sieves all particles larger than their specified pore sizes.
- The water flows through them quite quickly.
- They can withstand a pressure difference across them of around 2 to 5 atmospheres.
- Easy to clean «backwash».
Water can be stored in the rear tanks for anywhere from a few days to several months to create a natural biological purification process. This process is essential, especially if the treatment is carried out through slow sand filters. Storage tanks also ensure water sustenance during brief periods of drought/water scarcity or allow maintenance of the water supply system at some point in temporary incidents of contamination in the supply system.
There are different desalination techniques. In the early stages, they were based on the distillation or thermal evaporation of seawater on a large scale. At the time of exporting this method to other regions of the world, this process was interrupted due to the high cost of the process.
Membrane technologies are used to desalinate brackish water and not so much for seawater. The higher the dal content in water, the more expensive the procedure is.
Various membrane technologies can remove microorganisms and many naturally occurring contaminants. Additionally, it has lower capital costs as it requires much less energy compared to thermal systems.
Thermal desalination systems are distinguished by producing water with reduced salt content.
Multistage Flash (MSF) distillation
Distillation is the most established desalination process. MSF technology is used for seawater desalination. Electricity and potable water production applications are carried out using thermal desalination processes. Thermal desalination is not compatible with RO or reverse electrodialysis (EDR) processes.
Improvements in MED and VC technologies have led to lower costs, making these processes profitable for MSF distillation.
80% of desalination capacity globally is through reverse osmosis (RO) and multi-stage flash distillation (MSF).
Multiplate-effect distillation (MED)
One of the advantages of the multi-effect distillation process is that it can be applied in different capacities. Over the years, continuous technological advances increase production capacity.
This process is normally used for small and medium-scale seawater desalination plants.
There are two methods:
Mechanical vapor compression (MVC).
Thermal vapor compression (TVC).
There are different types of constructions in vapor compression plants.
In these systems, unwanted ions are removed through columns packed with zeolite or ion exchange resin. The widely used water softening case removes Ca2+ and Mg2+ ions with Na+ or K+ ions.
In addition, this process is also used to prevent the passage of heavy metals such as mercury, lead, and arsenic.
This process is similar to «ion exchange» treatment but differs in the use of cation and anion-selective membranes to separate charged ions.
In this process, regular automatic polarity inversion is used to reduce the soiling process.
Today, Reverse Osmosis (SWRO & BWRO) is the most extensive and advanced desalination system in the world, and it consists of applying pressure to a salt water solution and making it go through a semi permeable membrane that will allow the water to got throught but will stop the dissolved salts in it. The water will flow from the side where the salt concentration is higher to the side where the concentration is lower, resulting in a solution that is minimized in favor of the freshwater.
In Nature, osmosis is a phenomenon that consists in a semi-permeable membrane separating two solutions with the same solvent through which the solvent will pass through leaving the dissolved salts behind, until both sides of the membrane have solutions with the same amount of concentration, this process is carried out without the input of external energy. However, reverses osmosis can be performed thanks to the external application of energy in the form of pressure, overcoming the natural osmotic pressure of the solution.
The pressure required for this water treatment is considerably high. Arflu offers a wide range of Solutions for reverse osmosis plants, especially for the high-pressure stage, where most equipment including but not only valves are made in special materials against corrosion such as Duplex and Super Duplex materials. Being the pressure high as mentioned earlier, note reverse osmosis demands ANSI Class 600 or PN 100 rating valves since working pressure is about 65 to 80 bar.
The most used valve type in the high-pressure stage are the plug valves made in Duplex & Super Duplex. Arflu manufactures RO Plug valves for different service applications into the high-pressure stage of SWRO plants: high-pressure pump discharge isolating and control plug valves, isolating to ERD, CIP service RO plug valves and others.
Arflu has developed some unique (patented) solutions such as the 3 ways plug valve that replaces 2 isolating valves to ERD and CIP & Chick Change solution that allows to replace Teflon sleeve fast, easily and without any technical assistance nor special tooling is required. Arflu also manufactures other valves solutions for Reverse Osmosis such as Ball, Check and Globe Control valves. Would you like to know more about Arflu’s plug valves and its possibilities for desalination plants? Click here.
Nano Filtration (NF)
For nanofiltration, a membrane with a pore size of 0.5 and 2 nm and operating pressures between 5 and 40 bars are necessary. This process is used for fluids containing organic molecules, sugars, and multivalent salts.
The nanofiltration membrane for negatively charged ions.
The uncharged nanofiltration membrane rejects dissolved matter. For positively charged ions, rejection is affected by membrane charge and according to the membrane fouling mechanism.
The NF process is used in many industries. Power consumption can be affected by temperature, pressure, and pH. There has been a lot of research to see the effect of operating temperature on neutral solute and water transport across NF membranes. Mass transfer and pressure drop are affected by operating parameters and also by power consumption.
Membrane Distillation (MD)
It is currently in limited commercial use. Distillation uses a synthetic water-repellent membrane to allow water vapor to drift through the pores of the membrane and prevent solution.
Membrane distillation uses the temperature difference across a membrane to vaporize water from a brine solution and condense the clear condensate on the cooled side.
Fordward Osmosis (FO)
Forward osmosis is a very recently developed process. It is used for the desalination of water using a ratio between the variation of the value of a magnitude in two nearby points and the distance that separates them. If the feed is on one side of the membrane, on the other side of the membrane is the solution with the highest osmotic pressure. In a natural migration process, water from the feed solution will flow into the extraction solution without using added external pressure.